A surface analyzer is an apparatus in which a sample to be analyzed is irradiated with accelerated proton beams. A scattering pattern of the proton beams caused by collisions with atoms in the sample which decelerate the protons is generated and the energy distribution of the decelerated proton beams is measured to identify the species of atoms on the surface of the sample, as well as to determine their proportions.
The crystalline structure of the bulk of a sample can be analyzed by X-ray diffraction. Techniques such as electron diffraction are also available for examining the cyrstalline structure of a near-surface area of the sample. None of these methods, however, are capable of providing the distribution of elements on the very surface of the sample, for example on the topmost or within the top two atomic layers.
The present inventors developed a technique called PELS (proton energy loss spectroscopy) as a method for measuring the elements on the topmost or within the top two atomic layers of a sample. PELS is a new technique of measurement and its operating principles will be briefly described.
Suppose, as shown in FIG. 3, a proton of mass number m, moving with velocity U hits an atom of mass M at rest. After collision, the proton is scattered by the atom and glances off at velocity V along a path deflected from its original path by an angle .theta. while the atom moves at velocity W in another direction defined by angle .PHI.. Since momentum is conserved in both the x- and y-directions, the following two equations are established: EQU mU=mV cos .theta.+MW cos .PHI. (1) EQU O=mV sin .theta.+MW sin .PHI. (2)
If the collision is completely elastic, kinetic energy is conserved so that; EQU (1/2)mU.sup.2 =(1/2)mV.sup.2 +(1/2)MW.sup.2
Eliminating W from Eqs. (1) to (3), EQU (.GAMMA.+1)V.sup.2 -2U cos .theta.V-(.GAMMA.-1)U.sup.2 =0 (4)
Then, ##EQU1## where EQU .GAMMA.=(M/m) (6)
The minus sign in Eq. (5) indicates scattering at angle (.pi.-.theta.). Only the plus sign should be taken to consider scattering at angle .theta..
The proton will lose part of its energy as a result of scattering. For the same scattering angle a collision with a lighter atom will cause greater energy loss than a collision with a heavier atom. Therefore, by measuring the energy loss occurring in the proton, the identity of the atom against which it collided can be determined.
If the kinetic energy of the proton before a collision is written as E.sub.0, EQU E.sub.0 =(1/2)mU.sup.2 ( 7)
and if the energy of the proton after collision is written as E.sub.1, E.sub.1 is always smaller than E.sub.0. The ratio of E.sub.1 /E.sub.0 is called the coefficient of attenuation K. The following equations will be established: EQU E.sub.1 =KE.sub.0 ( 8) ##EQU2##
It is not .theta. but .GAMMA. which is a variable because .theta. is uniquely determined by the experimental apparatus employed.
The present inventors first developed a PELS apparatus of low scattering angle (.theta..congruent.0) as disclosed in Unexamined published Japanese application No. 180945/1984 (published Oct. 15, 1984) and 151958/1986 (published July 10, 1986).
The low scattering-angle apparatus has the disadvantage that it is highly susceptible to the surface state of a sample to be analyzed as will be apparent from FIG. 12. In addition to single scattering, double scattering might also occur on account of asperities on the surface of the sample. Another disadvantage of the use of a low scattering-angle is its low resolution since K is not highly sensitive to .GAMMA. as suggested by Eq. (9).
The operating theory of PELS is basically set forth in Eq. (9) but this assumes attenuation by single scattering and is not valid if multiple scattering occurs.
Low scattering angles were selected for the simple reason that they produce high proton yield. FIG. 13 shows the proton yield vs. scatterning angle .theta. for Au and Si. The proton yield as relative against the scattering angle is determined by geometric factors and will not depend upon the physical properties of a specific atom. As shown in FIG. 13, a maximum yield is also attained at .theta.=180.degree.. Eq. (9) shows that at .theta.=180.degree., the highest resolution can be attained with respect to .GAMMA.. In this case where .theta.=.pi., the coefficient of attenuation K can be rewritten as: ##EQU3##
The parameter .GAMMA. denotes the ratio of the mass, M, of the atom to the mass, m, of the proton. If the slight difference between the mass of the proton and the atomic mass unit is disregarded, .GAMMA. may safely be referred to as the mass number of the atom of interest.
For various elements, the .GAMMA. values and hence K values can be determined. The mass numbers, as defined above, of atoms are listed below together with corresponding K values:
Al: .GAMMA.=26.98, K=0.8621 PA1 Ga: .GAMMA.=69.72, K=0.94422 PA1 As: .GAMMA.=74.9, K=0.94799.
In this way, the K values of all elements of atoms can be easily calculated.
The foregoing discussion can be summarized as follows. If the kinetic energy, E.sub.1, of a proton after collision is measured, K can be determined by calculating the ratio of E.sub.1 to E.sub.0. This leads to the determination of .GAMMA. value and hence to the identification of the atom against which the proton collided. Then, the abundance of that particular atom in a sample of interest can be determined from the energy spectrum. In most cases, E.sub.0 is selected at about 100 keV.
The principles of PELS are very simple as described above. In order to enable measurements by PELS, the proton must be scattered only once. Instead of direct measurement of E.sub.1, the energy loss .DELTA.E[=(1-K)E.sub.0 ] may be measured. The name "PELS" derives from this measurement of the energy loss distribution of a proton.
As shown in FIG. 4, when protons having mass m are scattered by a heavy atom M, an area where no protons exist will occur in the forward direction and this is generally referred to as a shadow cone. Few of the protons travelling a far distance from the atom M are scattered and those travelling near the atom M are highly likely to be scattered. This is the mechanism behind the formation of a shadow cone.
If an x-axis is assumed to lie in the direction in which a proton travels and a y-axis is assumed to lie in a direction perpendicular to that direction of travel, a repelling Coulomb force acting between the atom and the proton, will produce a shadow cone with a shape that can be expressed by: ##EQU4## where e is the elementary quantity of a load, z is a charge on the proton, and Z is a charge on the atom (with e being a unit).
By using the shadow cone, one can identify the atoms in the top monolayer of a sample, as is clear from the following discussion. Suppose proton beams are directed perpendicularly to the surface of a GaAs sample as shown in FIG. 5. The protons are scattered both by Ga and by As, so two peaks occur in the distribution of proton energy as shown in FIG. 6.
If proton beams are impinged on the sample at an angle as shown in FIG. 7 such that the atoms of the element in the next to the top layer are located within the shadow cones created by the atoms of the element in the topmost layer, the distribution of proton energy loss will have only the peak corresponding to the atoms in the topmost layer. With reference to FIG. 8, only the Ga peak appears. This result shows that Ga atoms are present in the topmost layer of the sample.
When proton beams are launched into a sample, proton energy loss occurs as a result of collisions not only with atoms but also with electrons. The energy loss due to collisions with electrons is proportional to the distance the proton travels in the sample. This will be explained more specifically with reference to FIG. 9.
If proton beams are launched into a sample at an angle .theta./2 with respect to surface layers, the proton energy loss differs between two cases. One case is when an incident proton is scattered at point J in the topmost layer and is reflected at angle .theta./2, and the other case is where the incident proton is scattered at point G in the next to the top (second) layer. The differential energy loss is expressed as: EQU .DELTA.E=2dS cos ec(.theta./2) (12)
where d is the distance between two layers and S is the stopping power of electrons. The value of .DELTA.E increases with decreasing .theta.. Even if .theta.=.pi., .DELTA.E is 112 eV assuming that d=5.6 .ANG. and S=10 eV/.ANG..
The above discussion shows that even in the case of normal (.theta.=.pi.) launching of proton beams, the energy lost by the protons scattered from the topmost layer differs by about 100 eV from the loss due to scattering in the second layer. In other words, even if the atom of mass M which is the principal factor of proton scattering is the same, protons will lose energy by different degrees in the topmost and second layers.
By decreasing .theta., .DELTA.E can be sufficiently increased to provide for distinction between proton scattering by dissimilar atoms in the topmost and second monolayers.
FIG. 10 shows the general layout of a prior art PELS measuring system in the case where the scattering angle .theta. is 180.degree.. Protonic beams extracted from an ion source A are subjected to mass separation in a magnet B. Only monovalent proton ions are selected and introduced into an acceleration tube C for acceleration.
The proton beams acquiring a kinetic energy of E.sub.0, which is the sum of the extraction energy Eex and the acceleration energy Eacc provided by the acceleration tube C, will impinge on a sample .SIGMA.. The protons are scattered by atoms in the surface of the sample .SIGMA..
Only the protons scattered at angle .theta.=.pi. will travel backward through the accelerating tube. Those which were scattered at angle .theta..noteq..pi. will collide against the wall of the chamber, make transition from the ionic to the neutral molecule (H.sub.2) form, and be discharged from the chamber. The protons scattered at angle .theta.=.pi. and that travel backward through the accelerating tube are decelerated. In other words, the accelerating tube now works as a decelerating tube.
The decelerating energy Edec is equal to the accelerating energy Eacc: EQU Eacc=Edec (13)
One of the advantages of the case where .theta.=.pi. is that a common tube can be used both as an accelerating tube C for accelerating proton beams and as a decelerating tube D for decelerating proton beams.
The decelerated proton beams are bent by 90.degree. with a magnet F. The protons are thereafter launched into an analyzer G at an angle. Two magnets E and F are necessary in order to converge the proton beams whose energy has a variance due to scattering loss .DELTA.E.
The convergent proton beams are subjected to energy detection in the analyzer G. A voltage V.sub.o is applied between two parallel electrode plates. A proton launched into the analyzer through a slit will travel on a parabolic path and fall into either one of the channels in a microchannel plate H. The channel into which the proton has fallen will indicate the distance L from the slit to the falling position of the proton, and hence the kinetic energy of the proton at the time that it was launched into the analyzer.
The distance L in the analyzer G increases as the kinetic energy of the proton increases. If the kinetic energy of the proton projecting onto the slit in the analyzer G is written as Ea, the angle formed between the incident proton beam and the electrode plate with the slit as .PSI., the distance between the two parallel electrode plates as h, and the electrostatic voltage applied between the plates as V.sub.o, then L is expressed as: ##EQU5## The distribution of distance L indicates the distribution of proton energy Ea, thereby enabling the measurement of proton energy distribution.
All the components of the system shown in FIG. 10 are placed in high vacuum. The sample .SIGMA. must be in an ultrahigh vacuum. If this requirement for vacuum is not met, protons impinging on the molecules of a gas will lose energy and the scattering loss .DELTA.E due to the sample .SIGMA. cannot be correctly determined.
For the sake of simplicity, the vacuum chamber and the evacuation unit are omitted from FIG. 10.
The change in the energy of a proton is described hereinafter with reference to FIG. 11 which shows the potential energy of the proton as a function of its position during movement (assumed to be left to right in FIG. 11).
A proton ion is extracted from the ion source at an extracting volage of Vex. The proton has a charge q, which produces a potential energy of qVex. When the proton leaves the ion source, this energy changes to kinetic energy.
The proton is accelerated in the acceleration tube C at an accelerating voltage of Vacc. The proton emerging from the acceleration tube toward the sample .SIGMA. has a kinetic energyof q(Vex+Vacc), which is equivalent to E.sub.o and is approximately 100 keV.
The proton beam impinges on the sample and is scattered therefrom, losing an energy of .DELTA.E. The scattered proton beam travels backward through the accelerating tube and loses kinetic energy equal to qVacc.
The proton beam, when it enters the analyzer G, has a kinetic energy Ea expressed as follows: EQU Ea=qV.sub.o -.DELTA.E (15)
where V.sub.o is equal to the extracting voltage Vex or may be described as the energy of proton launching into the analyzer G when the energy loss is assumed to be zero.
The value of Ea is set to be about 0.5 keV and the energy of the scattered proton Es, which is written as: EQU Es=q(Vex+Vacc)-.DELTA.E (16)
is set to be about 100 keV.
Theoretically, Ea should be a variable. In practice, the atom to be analyzed is preliminarily determined and Vacc is determined in such a way that Ea associated with this atom will be about 0.5 keV.
FIG. 14 is a schematic view showing the general layout of PELS equipment. Proton beams issuing from an ion source are focused by an einzel lens, deflected by a magnet and accelerated by an accelerating/decelerating tube. A sample to be analyzed is set in an ultrahigh vacuum chamber and can be handled with a manipulator. The accelerated proton beams are converged with a Q lens and subsequently impinge on the sample in the ultrahigh vacuum chamber. Among the protons scattered from the sample surface, those scattered at .theta.=180.degree. emerge from the ultrahigh vacuum chamber and are decelerated. The decelerated proton beams are bent by 180.degree. by magnets 1 and 2 and enter an analyzer for measurement of energy loss .DELTA.E.
The foregoing is intended to explain the principles of PELS, the composition of PELS equipment, and the mechanism of its action. The present invention relates to an improvement of a section of PELS equipment for measuring the energy loss, .DELTA.E, of a proton beam.
In the prior art system, a dc voltage V.sub.o is applied between two parallel electrode plates in such a way that a proton will fly on a parabolic path and the distance it travels is used as a basis for measurement of proton energy Ea in the analyzer G which depends on voltage for energy measurement.
The analyzer shown schematically in FIG. 10 employs two magnets. A prior art analyzing system using two magnets is shown schematically in FIG. 15. A proton beam scattered from the sample and decelerated by a decelerating tube D is bent by 90.degree. in magnet E and bent by another 90.degree. in magnet F. This can be realized by arranging the two magnets in such a way that the angle of intersection between the beam and the oblique side of each magnet is 45.degree..
Even if two protons have different energies, they have the same cyclotron angular frequency in a magnetic field. In addition, the kinetic energy of protons is invariable in a magnetic field. Therefore, if a proton is supposed to move on a circular orbit, the radius of the circle is proportional to its velocity, and the time required to travel a given portion (arc) of the circle is the same if the central angle is the same.
The advantage of using two magnets having an oblique angle of 45.degree. is that protons having different energies can be converged into a single fine beam. This beam is launched into an electrostatic analyzer G through a fine slit at an angle of 45.degree..
The distance L in the analyzer is determined by Eq. (14).
The energy measuring system of the type described above is adopted in the PELS equipment shown in co-assigned Japanese Patent Application No. 164299/1986 (filed July 12, 1986). An analyzing system using one magnet (also prior art) is shown schematically in FIG. 16. This system uses one magnet with an oblique angle of 45.degree.. When scattered protons are launched into this magnet at 45.degree., those having the smaller energy will travel on a circular path with a small radius of curvature, and those having the greater energy will travel on a circular path with a large radius of curvature.
In this way, proton beams can be separated spatially. The separated beams are launched into a wide electrostatic analyzer G through an elongated slit. As in the case of a two magnet analyzer, the distance L travelled by protonss in the analyzer is determined by Eq. (14).
The obvious advantage of the system shown in FIG. 16 is that the number of magnets needed is one, rather than two. An energy measuring system of the single magnet type described is shown in co-assigned Japanese Patent Application No. 299269/1986 (filed Dec. 16, 1986).
The prior art system for measuring the proton energy depends on an electrostatic voltage for changing the direction of the travel of protons and this has caused several problems. For example, in the electrostatic analyzer, the direction in which voltage is applied is not perpendicular to the direction of motion of the proton beams. A faster proton beam having a long flight will fly to a point close to the positive electrode plate. Since this should not happen, the distance between the two electrodes must be increased but then the size of the electrostatic analyzer is increased. Not only does this increase the cost of the analyzer but also the load on the evacuation unit is increased and additional vacuum pumps must be installed.
Moreover, both the parameters of the magnetic field H of a magnet and the voltage V.sub.o of the electrostatic analyzer G must be adjusted on the prior art systems. The need to adjust these parameters introduces complexity. For instance, if the magnetic field of magnet E is increased in the measuring system shown in FIG. 15, the beam emerging from this magnet has been bent 90.degree. but at the same time, it has been displaced more outwardly than when the magnet is small. Unless the magnetic field of magnet F is increased correspondingly, the beam emerging from magnet F will be offset too much to pass through the slit 5.
In the case of the one-magnet system shown in FIG. 16, a wide electrostatic analyzer is necessary. Furthermore, the width of the microchannel plate must also be increased. This leads to a very expensive and hence uneconomical analyzer.